Both magnetic resonance tomography (MRT) and positron emission tomography (PET) are imaging methods that are suitable for displaying the interior of the human body and, particularly in the case of PET, for displaying biochemical processes. There has recently been interest in combining MRT and PET with one another in a unit in order to be able to apply both imaging modalities to the same patient simultaneously or shortly after one another.
It is proposed to this end to provide a possibly removable PET detector additionally inside the magnet arrangement, customary in a magnetic resonance apparatus, composed of basic field magnet and gradient coils. A radiofrequency coil arrangement is to be arranged coaxially for the MRT pictures inside this PET detector arrangement, which is mostly surrounded by a radiofrequency shield.
Both imaging methods, and thus both arrangements share the same isocentric region, that is to say both the PET detector arrangement and the radiofrequency coil arrangement require the same isocentric space in the plane perpendicular to the longitudinal axis of the magnetic resonance apparatus, which is frequently also denoted as the Z-axis, the isocenter being located at Z=0.
To this end, it has been proposed to arrange the PET detector arrangement inside the patient receptacle of the magnet of the magnetic resonance apparatus, the radiofrequency coil arrangement, in turn, being inserted into the annular PET detector arrangement. However, a few problems arise in this case. Firstly, the radiofrequency coils and the PET detectors interfere mutually with one another, since the currents generate inside the PET detector arrangement interference fields that can be captured by the radiofrequency coil and possibly lead to interference signals. In order to avoid this interference, it is proposed to provide a radiofrequency shield at least between the radiofrequency coil arrangement and the PET detector arrangement, such that the respective interference fields are shielded against one another. In this case, the entire PET detector arrangement can also be surrounded by such a radiofrequency shield in the manner of a Faraday cage.
A further problem is that the patient receptacle is extremely reduced in the case of such a design of onion-skin type. The inside diameter of the radiofrequency coil arrangement remaining for the patient is then very small, and so psychological effects such as, for example, claustrophobia, and problems based on a high body volume can greatly stress the patient by reducing comfort.
If such a radiofrequency shield is provided, the problem is additional rendered more acute by virtue of the fact that the conductors of the radiofrequency coil arrangement can no longer be arranged directly in the vicinity of this radiofrequency shield, since a certain reverse field flux space is required for the field lines of the radiofrequency field to be able to close. Consequently, the conductors of the radiofrequency coil arrangement should exhibit a certain distance from a radiofrequency shield, and this leads to a further constriction of the space that remains for the patient.